The Sunday Eucharist replaces the Sabbath

If, therefore, those who were brought up in the ancient order of things have come to the possession of a new hope, no longer observing the Sabbath, but living in the observance of the Lord’s Day, on which also our life has sprung up again by Him and by His death—whom some deny, by which mystery we have obtained faith, and therefore endure, that we may be found the disciples of Jesus Christ, our only Master—how shall we be able to live apart from Him, whose disciples the prophets themselves in the Spirit did wait for Him as their Teacher? And therefore He whom they rightly waited for, being come, raised them from the dead.

Ignatius of Antioch
Epistle to the Magnesians
Chapter IX

Eucharist—the medicine of immortality

Especially will I do this if the Lord make known to me that you come together man by man in common through grace, individually, in one faith, and in Jesus Christ, who was of the seed of David according ot the flesh, being both the Son of man and the Son of God, so that you obey the bishop and the presbytery with an undivided mind, breaking on and the same bread, which is the medicine of immortality, and the antidote to prevent us from dying, but which causes that we should live forever in Jesus Christ.

Ignatius of Antioch
Epistle to the Ephesians
Chapter XX